1. Field of the Invention
This invention relates to environmental screening apparatuses such as those that subject mechanical electromechanical and electrical devices to a variety of environments designed to reveal defects in the devices.
2. Description of the Prior Art
Environmental stress screening is a process by which environmental stresses are applied to recently manufactured devices to disclose defects in the devices that would not otherwise be exposed by more common techniques such as visual examination. If defects go undiscovered at the point of manufacture they may eventually reveal themselves in the field where repair of the device can cost hundreds of times the cost of repair at the point of production.
Over the past few years environmental stress screening of electrical devices has reached the state where the process consists of an average of 12 minutes of random vibration and of 20 cycles of temperature changes. Where feasible, electrical devices are cyclically energized by repetitively turning the devices on and off at the same or varying power levels.
Apparatuses for vibration screening devices often require custom made fixtures to attach the devices to the apparatuses. As the size and shape of the device to be tested changes, the fixture must be changed or redesigned accordingly. The expense of these changes is ultimately added to the production cost of the device.
Some prior art vibration screening apparatuses suffer not only from the above shortcoming but also are ineffective in transmitting controlled vibration to the device to be tested. This may be due to an inadequate area of force transmittal between the fixture and the device or may be due to a mismatch between the mass of the fixture and the production device or even a combination of these.
The problems of the prior art vibration screening apparatuses are exasperated when large items, such as those greater than or equal to 12 inches on a side, are tested. Frequently it is necessary to fabricate custom made fixtures for these items and it is common that a single vibration driver will not be suitably effective in vibrating the item.
Multiple drivers can be used but of course this adds further to the cost of the item. As a result, larger items are often exempted from vibration screening or they receive vibration exposures very much reduced in effectiveness.
Temperature cyclical screening facilities typically use air to transfer heat to and from the item to be tested. Though it is now becoming more widely understood that to effectively temperature screen electrical devices rapid temperature changes approaching 20 degrees Celsius per minute are needed, most air chambers provide temperature change rates of only about 2.5 degrees Celsius per minute.
For small items, it is possible to achieve rapid temperature cycling by using hot-air and cold-air climatic chambers and moving the item from one chamber to the other. For larger items the difficulty in changing chambers is outweighed by the advantage, so that the reduced efficiency of a slow rate of temperature change is accepted.
In many cases it is desirable to energize an electrical item while it is being subject simultaneously to vibration and temperature environments. When electrical items are shuffled between chambers, cyclicly energizing the item can prove to be difficult. Electrical connections must be sufficiently long and there is the potential for electrical shock or accidental circuit grounding.
If an electrical device is energized in a hot air environment it can sometimes generate local areas of heating that exceed the proposed upper temperature limit of the device. Even if forced convection is used, the air can be insufficient in carrying heat away from the locally heated regions so that destruction of both flawed and flawless devices may occur.
Under the ideal test conditions, a device will be subjected to conditions that approach as near as possible those conditions the device is expected to go through during its normal duty cycle. It is also desirable to subject devices to certain extremes beyond those of normal environments to accelerate the "surfacing" of production flaws that would not otherwise be detected.
In either case test screening apparatuses should be able to subject a device to multitude of test screenings at once.
Some prior art schemes exist in which temperature cycling and vibration are combined. Others combine device energization and temperature cycling. Generally, both of these established methods suffer from the shortcomings described above.
A need therefor exists for an environmental screening apparatus that subjects a device to simultaneous temperature, vibrational and, where applicable, energization cycles while at the same time avoids the expensive fixtures, multiple chambers, and ineffectual heat transfer of the prior art.